CODESAMPLE

Visitor - C

Visitor C

The Visitor pattern allows you to add new operations to a hierarchy of objects without modifying the objects themselves. It achieves this by defining a separate “visitor” class that implements the operations, and then having each element in the hierarchy “accept” the visitor, allowing it to perform its operation. This promotes the Open/Closed Principle.

In this C implementation, we have an abstract Animal structure and concrete types like Dog and Cat. The Animal defines an accept method taking a Visitor pointer. Each concrete animal type implements the accept method to call the appropriate visit method on the visitor, passing itself as an argument. The Visitor abstract structure defines visit methods for each concrete animal type. A concrete visitor, AnimalSoundVisitor, demonstrates how to add a new operation (making a sound) without altering the animal classes. This uses function pointers to achieve polymorphism, a common C technique.

#include <stdio.h>

// Forward declaration of Animal
typedef struct Animal Animal;

// Visitor interface
typedef struct Visitor Visitor;
struct Visitor {
    void (*visitDog)(Animal* animal);
    void (*visitCat)(Animal* animal);
};

// Animal base class
struct Animal {
    void (*accept)(Animal* animal, Visitor* visitor);
};

// Concrete animal: Dog
typedef struct Dog Dog;
struct Dog {
    Animal base;
};

void dogAccept(Animal* animal, Visitor* visitor) {
    visitor->visitDog((Dog*)animal);
}

// Concrete animal: Cat
typedef struct Cat Cat;
struct Cat {
    Animal base;
};

void catAccept(Animal* animal, Visitor* visitor) {
    visitor->visitCat((Cat*)animal);
}

// Concrete visitor: AnimalSoundVisitor
typedef struct AnimalSoundVisitor AnimalSoundVisitor;
struct AnimalSoundVisitor {
    Visitor base;
};

void dogVisit(Animal* animal) {
    printf("Woof!\n");
}

void catVisit(Animal* animal) {
    printf("Meow!\n");
}

AnimalSoundVisitor createAnimalSoundVisitor() {
    AnimalSoundVisitor visitor;
    visitor.base.visitDog = dogVisit;
    visitor.base.visitCat = catVisit;
    return visitor;
}

// Example Usage
int main() {
    Dog dog;
    dog.base.accept = dogAccept;

    Cat cat;
    cat.base.accept = catAccept;

    AnimalSoundVisitor soundVisitor = createAnimalSoundVisitor();

    dog.base.accept(&dog, &soundVisitor.base);
    cat.base.accept(&cat, &soundVisitor.base);

    return 0;
}